This invention relates to burners for producing boules of fused silica glass, such as, high purity fused silica glass (HPFS glass) and ultra low expansion glass, from halide-free, silicon-containing (HF-SC) starting materials, such as, octamethyl-cyclotetrasiloxane (OMCTS).
Fused silica glass is made commercially by the assignee of this invention using furnaces of the type shown in commonly-assigned PCT Patent Publication No. WO 97/10182, the contents of which are incorporated herein by reference. FIG. 7 hereof is a copy of FIG. 4 of WO 97/10182. In overview, furnaces of this type utilize flame hydrolysis to produce and deposit fine silica particles (silica soot) on a planar surface (e.g., a layer of bait sand) which is then consolidated into a solid glass boule. More particularly, furnaces of this type operate at a sufficiently high temperature so that the consolidation takes place essentially simultaneously with the depositing of the silica soot.
As shown in FIG. 7, furnace 100 includes crown 12 which carries a plurality of burners 14 which produce the silica soot which is collected to form boule 19, which typically has a diameter on the order of five feet (1.5 meters). The present invention is concerned with the structure and operation of burners 14.
In the past, burners 14 have been unable to deposit soot in a sufficient manner at distances greater than six inches from the burner face, which has meant that the maximum boule thickness has been six inches. To meet the demand for fused silica products, especially, HPFS glass, it would be desirable to produce boules having a thickness greater than six inches, e.g., boules having a thickness of 8-10 inches. The present invention is directed to providing burners capable of producing such boules.
Commonly-assigned U.S. Pat. No. 5,599,371 (the ""371 patent) describes burners suitable for use in producing preforms for optical waveguide fibers from HF-SC starting materials. As explained in that patent, prior burners used to produce preforms from starting materials which contained halides (hereinafter the xe2x80x9chalide burnerxe2x80x9d) had five concentric gas-emitting regions: 1) a central region (fume tube) which emitted a mixture of a halide-containing/silicon-containing starting material (e.g., SiCl4) and an inert gas, 2) an innershield region which emitted oxygen, 3) a third region which emitted a mixture (premix) of a combustible gas and oxygen, 4) a fourth region which also emitted a mixture (premix) of a combustible gas and oxygen, and 5) an outershield region which emitted oxygen.
As explained in the ""371 patent, when an HF-SC starting material was substituted for the halide-containing/silicon-containing starting material previously used to produce preforms, it was found that the gases emitted from the various regions of the burner had to be changed. In particular, instead of the above gases, the five concentric gas-emitting regions of the burner of the ""371 patent emitted the following gases: 1) the fume tube emitted a mixture of the HF-SC starting material and oxygen plus, optionally, an inert gas, 2) the innershield emitted an inert gas, 3) the third region emitted oxygen, 4) the fourth region emitted oxygen, and 5) the outershield emitted a mixture (premix) of a combustible gas and oxygen.
In the course of the development of the burner of the present invention, an attempt was made to use a burner having the gas arrangement of the ""371 patent. It was surprisingly found that although such a burner works successful in producing optical waveguide preforms from HF-SC starting materials, it does not work particularly well in producing boules from such materials. In particular, it does not work successfully in producing thick boules from such starting materials.
Rather, in accordance with the invention, it was found that to successfully make a thick boule from an HF-SC starting material, the burner has to have the following concentric regions emitting the following gases: 1) a central region (fume tube) which emits a mixture of an HF-SC starting material and an inert gas, 2) an innershield region which emits oxygen, 3) a third region which emits a mixture (premix) of a combustible gas and oxygen, 4) a fourth region which emits a mixture (premix) of a combustible gas and oxygen, 5) a fifth region which emits a mixture (premix) of a combustible gas and oxygen, and 6) an outershield region which emits oxygen.
The foregoing burner history illustrates the difficulties in predicting whether a particular gas arrangement will work with a particular starting material (i.e., a halide-containing starting material versus a halide-free material) to produce a particular product (i.e., an optical waveguide preform versus a thick boule). Thus, an oxygen/premix/premix/oxygen arrangement surrounding a fume tube carrying a halide-containing raw material worked to produce preforms in the halide burner referred to above, but did not work when the raw material was halide free. Similarly, an inert gas/oxygen/oxygen/premix arrangement surrounding a fume tube carrying a halide-free raw material worked to produce preforms as disclosed in the ""371 patent, but was found in the course of the development of the burner of the present invention to not work in the production of thick boules. As described in detail below, to produce thick boules, an oxygen/premix/premix/premix/oxygen arrangement surrounding a fume tube carrying a halide-free raw material needs to be used.
In view of the foregoing, it is an object of the present invention to provide soot-producing burners that will produce thicker boules and thus increase the yield of fused silica glass and, in particular, high purity fused silica glass and ultra low expansion glass, produced in furnaces of the general type shown in FIG. 7. It is a further object of the invention to provide a silica soot deposition technique that produces fused silica that exhibits very high optical qualities and has a large cross section and thickness.
To achieve these and other objects, the invention in accordance with certain of its aspects provides a method for forming a silica-containing boule (19) comprising:
(a) providing a furnace (100) which comprises:
(i) a cavity (26);
(ii) at least one burner (14) which produces a stream of soot particles; and
(iii) a substantially planar surface (24) within the cavity (26) for collecting the soot particles to form the boule;
(b) providing a halide-free, silicon-containing material to the at least one burner; and
(c) collecting the soot particles to form the boule;
wherein the width of the stream of soot particles is controlled to enhance the efficiency of step (c).
In particular, the width of the stream of soot particles is controlled in accordance with the discovery, illustrated in FIG. 6, that a reduction in the width leads to enhanced efficiency of step (c). That a reduction in width has this effect is counterintuitive since a priori one would think that widening the stream, rather than narrowing it, would result in the laydown of more soot particles.
In quantitative terms, the width of the stream at the working distance is preferably less than 25 millimeters, most preferably, less than 12 millimeters, where the working distance (i.e., the distance between the burner face and the surface of the boule) is at least 150 millimeters and preferably at least 200 millimeters or more.
In accordance with other aspects, the invention provides a method for forming a silica-containing boule (19) comprising the steps of:
(a) providing a substantially planar surface (24);
(b) providing a soot-producing burner (14) having a burner face (13) that comprises first (1), second (2), third (3), fourth (4), fifth (5), and sixth (6) gas-emitting regions, the second region surrounding the first region, the third region surrounding the second region, the fourth region surrounding the third region, the fifth region surrounding the fourth region, and the sixth region surrounding the fifth region;
(c) providing a mixture comprising an inert gas and a halide-free, silicon-containing material to the first region;
(d) providing oxygen to the second region;
(e) providing a mixture of a combustible gas and oxygen to the third region;
(f) providing a mixture of a combustible gas and oxygen to the fourth region;
(g) providing a mixture of a combustible gas and oxygen to the fifth region;
(h) providing oxygen to the sixth region; and
(i) collecting silica-containing soot on the substantially planar surface to form the boule.
In accordance with other aspects, the invention provides a soot-producing burner comprising a burner face which comprises first (1), second (2), third (3), fourth (4), fifth (5), and sixth (6) gas-emitting regions, the second region surrounding the first region, the third region surrounding the second region, the fourth region surrounding the third region, the fifth region surrounding the fourth region, and the sixth region surrounding the fifth region, wherein:
(a) the first region emits a mixture of a halide-free, silicon-containing material and an inert gas;
(b) the second region emits oxygen;
(c) the third region emits a mixture of a combustible gas and oxygen;
(d) the fourth region emits a mixture of a combustible gas and oxygen;
(e) the fifth region emits a mixture of a combustible gas and oxygen; and
(f) the sixth region emits oxygen.
In accordance with certain preferred embodiments of the invention, the radial spacing between the third, fourth, fifth, and sixth regions is substantially the same.
In accordance with other preferred embodiments, the first, second, third, fourth, fifth, and sixth regions have the following forms at the burner""s face: the first region is in the form of an open disc or tube, the second region is an annular ring, and the third, fourth, fifth, and sixth regions are each a ring of orifices.